Enhanced translation of probe molecules in supercooled o-terphenyl: Signature of spatially heterogeneous dynamics?

Abstract

A holographic fluorescence recovery after photobleaching technique has been used to measure translational diffusion coefficients DT for four probes in supercooled o-terphenyl (OTP). DT values from 10−6 to 10−14 cm2/s were observed in the temperature range from Tg+8 K to Tg+90 K (Tg=243 K). In agreement with previous reports, the translational diffusion of probe molecules which are the same size as OTP molecules has a significantly weaker temperature dependence than T/η. However, as the size of the probe molecule is increased the temperature dependence of DT tracks T/η increasingly well. The transition between a weak temperature dependence to that of T/η occurs over only a factor of 3 in probe size. Previous work established that the rotational correlation times τc of these four probes in OTP tracks η/T. The product DTτc is independent of temperature for the largest probe but increases almost 2 orders of magnitude for the smaller probes as Tg is approached. A strong correlation is observed between this enhancement of translational diffusion and the KWW β values obtained from rotation measurements; probes with small β values show enhanced translation. These observations are qualitatively explained by spatially heterogeneous dynamics. The probes studied are rubrene, 9,10-bis(phenylethynyl)anthracene, tetracene, and anthracene.

Barrier softening near the onset of nonactivated transport in supercooled liquids: Implications for establishing detailed connection between thermodynamic and kinetic anomalies in supercooled liquids

“…Furthermore, it is the only existing theory that computes the cooperative length scale of those structural rearrangements (5-6 molecular diameters at T g ), without using adjustable parameters and that agrees with values found in many experiments [11,12]. Knowing the cooperative length allows one to predict deviations from the Stokes-Einstein relationship between liquid's viscosity and the particle's diffusion coefficient [10], as confirmed by several measurements [11,12].…”

“…Knowing the cooperative length allows one to predict deviations from the Stokes-Einstein relationship between liquid's viscosity and the particle's diffusion coefficient [10], as confirmed by several measurements [11,12]. Finally, it turns out that this length scale also dictates the spatial density of low energy excitations in glasses, observed at cryogenic temperatures [13], as well as the frequency of the Boson Peak and the thermal conductivity plateau in amorphous solids at low temperatures [14].…”

“…During the last decade, however, a variety of experiments using nonlinear spectroscopies [4], single molecule techniques and scanning microscopies [5,11] have revealed the dynamic heterogeneities that are intrinsic to this mosaic picture of the deeply supercooled liquid. Still more recently microscopic calculations based on the RFOT have made quantitative predictions both of the mean barriers [3] and the fluctuations of the barrier from region to region that give rise to nonexponential decay of measured time correlations [9].…”

Viscosity of deeply supercooled water and its coupling to molecular diffusion

“…SE relation is obeyed at sufficiently high temperature but violated around 1.3T g , where T g is the glass transition temperature, thus indicating decoupling between translational diffusion and viscosity. In contrast, it was observed for ortho-terphenyl (23,24,26) that rotational diffusion and viscosity remain strongly coupled (i.e., obey the SED relation) even very close to T g . A corollary is that translational and rotational diffusion decouple from each other at low temperature.…”

Aging of rotational diffusion in colloidal gels and glasses

“…According to these relations the translational and rotational diffusion coefficients are not independent and should both have a linear temperature dependence and be proportional to the inverse of the viscosity. While there have been some comprehensive studies on rotational motion of several molecular glass formers [5][6][7][8], there are fewer studies on rotational dynamics of glassy colloidal systems [9][10][11][12]. Here, we present the results of a systematic investigation of aging of the translational and rotational diffusion of charged colloidal platelets of Laponite studied for a wide range of concentrations and as a function of waiting time.…”